1. Field
The present disclosure relates generally to electronics, and more specifically to a DC offset cancellation circuit in a receiver.
2. Background
In a digital communication system, a transmitter processes traffic data to generate data chips and further modulates a local oscillator (LO) signal with the data chips to generate a radio frequency (RF) modulated signal. The transmitter then transmits the RF modulated signal via a communication channel. The communication channel degrades the RF modulated signal with noise and possibly interference from other transmitters.
A receiver receives the transmitted RF modulated signal, downconverts the received RF signal from RF to baseband, digitizes the baseband signal to generate samples, and digitally processes the samples to recover the traffic data sent by the transmitter. The receiver uses one or more downconversion mixers to frequency downconvert the received RF signal from RF to baseband. An ideal mixer simply translates an input signal from one frequency to another frequency without distorting the input signal. An ideal mixer receives an input RF signal in one input port and an LO signal from an LO generator in another input port and downconverts the input RF signal to a baseband signal by using the LO signal.
However, in an actual real world downconversion mixer, the LO signal from an LO signal generator may leak into the input port for the input RF signal. The LO signal leaks into the input port for the input RF signal through capacitive and substrate coupling (e.g., parasitic capacitance) that may exist between the input port for the input RF signal and the input port for the LO signal. Furthermore, the LO signal also may leak into an input port of a low noise amplifier (LNA) that may precede the downconversion mixer. The leakage LO signal produces a DC component in the output signal of the downconversion mixer. Basically, the DC component creates a DC offset in the output signal of the downconversion mixer, and the DC offset may eventually saturate an analog-to-digital converter (ADC) that digitizes the output signal of the downconversion mixer. Thus, the ADC will output incorrect values when the ADC is saturated by the DC offset.
Therefore, there is a need in the art for a circuit that minimizes the DC offset produced by leakage LO signals.